For many years, the design of concrete structures imitated the typical steel design of column, girder and beam. With technological advances in structural concrete, however, its own form began to evolve. Concrete has the advantages of lower cost than steel, of not requiring fireproofing, and of its plasticity, a quality that lends itself to free flowing or boldly massive architectural concepts. On the other hand, structural concrete, though quite capable of carrying almost any compressive load, is extremely weak in carrying significant tensile loads. It becomes necessary, therefore, to add steel bars, called reinforcements, to concrete, thus allowing the concrete to carry the compressive forces and the steel to carry the tensile forces.
Structures of reinforced concrete may be constructed with load-bearing walls, but this method does not use the full potentialities of the concrete. The skeleton frame, in which the floors and roofs rest directly on exterior and interior reinforced-concrete columns, has proven to be most economic and popular. Reinforced-concrete framing is seemingly a quite simple form of construction. First, wood or steel forms are constructed in the sizes, positions, and shapes called for by engineering and design requirements. The steel reinforcing is then placed and held in position by wires at its intersections. Devices known as chairs and spacers are used to keep the reinforcing bars apart and raised off the form work. The size and number of the steel bars depends completely upon the imposed loads and the need to transfer these loads evenly throughout the building and down to the foundation. After the reinforcing is set in place, the concrete, a mixture of water, cement, sand, and stone or aggregate, of proportions calculated to produce the required strength, is placed, care being taken to prevent voids or honeycombs.
One of the simplest designs in concrete frames is the beam-and-slab. This system follows ordinary steel design that uses concrete beams that are cast integrally with the floor slabs. The beam-and-slab system is often used in apartment buildings and other structures where the beams are not visually objectionable and can be hidden. The reinforcement is simple and the forms for casting can be utilized over and over for the same shape. The system, therefore, produces an economically viable structure. With the development of flat-slab construction, exposed beams can be eliminated. In this system, reinforcing bars are projected at right angles and in two directions from every column supporting flat slabs spanning twelve or fifteen feet in both directions.
Reinforced concrete reaches its highest potentialities when it is used in pre-stressed or post-tensioned members. Spans as great as one hundred feet can be attained in members as deep as three feet for roof loads. The basic principle is simple. In pre-stressing, reinforcing rods of high tensile strength wires are stretched to a certain determined limit and then high-strength concrete is placed around them. When the concrete has set, it holds the steel in a tight grip, preventing slippage or sagging. Post-tensioning follows the same principle, but the reinforcing tendon, usually a steel cable, is held loosely in place while the concrete is placed around it. The reinforcing tendon is then stretched by hydraulic jacks and securely anchored into place. Pre-stressing is done with individual members in the shop and post-tensioning as part of the structure on the site.
In a typical tendon tensioning anchor assembly in such post-tensioning operations, there are provided anchors for anchoring the ends of the cables suspended therebetween. In the course of installing the cable tensioning anchor assembly in a concrete structure, a hydraulic jack or the like is releasably attached to one of the exposed ends of cable for applying a predetermined amount of tension to the tendon. When the desired amount of tension is applied to the cable, wedges, threaded nuts, or the like, are used to capture the cable and, as the jack is removed from the tendon, to prevent its relaxation and hold it in its stressed condition.
Metallic components within concrete structures may become exposed to many corrosive elements, such ad de-icing chemicals, sea water, brackish water, or spray from these sources, as well as salt water. If this occurs, and the exposed portions of the cable or anchor suffer corrosion, then they may become weakened due to this corrosion. The deterioration of the anchor or tendon can cause the cables to slip, thereby losing the compressive effects on the structure, or the anchor can fracture. In addition, the large volume of by-products from the corrosive reaction is often sufficient to fracture the surrounding structure. These elements and problems can be sufficient so as to cause a premature failure of the post-tensioning system and a deterioration of the structure.
Several U.S. patents have considered the problem of anchor and cable corrosion. For example, U.S. Pat. Nos. 4,896,470, Felix L. Sorkin, Inventor, and 5,072,558, issued Dec. 17, 1991, Felix L. Sorkin, et al., Inventors, disclose cable tensioning anchor systems in which the metal anchor for the system is encapsulated in plastic and has a tubular portion extending outwardly towards the surface of the post-tensioned concrete body. A sealing cap is fitted to the end of the tubular portion of the plastic encapsulation to provide a fluid-tight seal for protecting the post-tensioned cable, anchor and tensioning wedges from exposure to the elements. Other prior art systems also exist in which the end of the post-tensioned cable is severed at a point inwardly from the outer surface of the post-tensioned concrete body and means are used to protect the cable end, anchor and tensioning wedges from exposure to the elements.
When using such prior art systems for corrosion protection of the tensioning cable and related apparatus, it is important that the cable be terminated at a point inboard from the outside surface of the post-tensioned concrete body. This requires that the end of the cable be cut just outboard of the tensioning wedges, and within the pocket or cavity formed by the anchor. The method most commonly used in the prior art for cutting the tensioned cable at this position is a conventional acetylene torch or cutting torch. However, use of the open flame of a torch creates some danger of fire or explosion in the surrounding environment. Also, cutting the metal cable with a torch at a point near to the tensioning wedges causes the cable and wedges to become heated and may result in a loss of temper of the metal or loosening of the post-tensioning wedges.
An alternate prior art method sometime used for cutting the cable in this area is a conventional electric saw. However, this requires that a portion of the slab or other concrete structure surrounding the anchor also be cut in order to reach the portion of the cable which is within the concave bowl of the anchor.
It is, therefore, the primary object of the present invention to provide a method and apparatus for severing the free end of a post-tensioned cable or tendon at a point near the tensioning wedges and within the depth of the bowl or pocket formed for the anchor member.
Another object is to provide such a method and apparatus in which the cable is cut without substantially heating the cable and tensioning wedges.
A still further object is to provide such a method and apparatus in which the cable can be cut at the desired location without damaging the post-tensioned concrete body.